Various heating devices have been proposed in the past for processing semiconductor wafers. These heating devices can be classified into two basic configurations. The first is a batch system in which multiple wafers (25 wafers to about 200 wafers) are loaded into a round tubular furnace and slowly heated to the desired temperature. Batch systems can be considered temperature equilibrium devices in that the furnace walls are approximately at the same temperature as the heating elements and the wafers being heated. Batch systems are typically referred to as "hot wall" systems since the furnace walls are at an elevated temperature. The primary advantage of a batch system is the ability to process many wafers at the same time therefore reducing the processing cost per wafer. The disadvantage of such a configuration, however, is the speed in which the wafer temperature can be elevated or cooled. The large thermal mass of a batch furnace prevents quick temperature changes and forces relatively long process times, ranging from about fifteen minutes to about five hours.
A second and newer approach to wafer heating is rapid thermal processing where a single wafer is heated in a small compartment using radiant energy as the energy source. For instance, such systems typically include a substrate holder for holding a semiconductor wafer and a light source that emits light energy for heating the wafer. During heat treatment, the semiconductor wafer is heated under controlled conditions according to a preset temperature regime. For monitoring the temperature of the semiconductor wafer during heat treatment, rapid thermal processing chambers also typically include temperature sensing devices, such as pyrometers, that sense the radiation being emitted by a semiconductor wafer at a selected band or wavelength. By sensing the thermal radiation being emitted by the wafer, the temperature of the wafer can be calculated with reasonable accuracy.
In alternative embodiments, instead of or in addition to using radiation sensing devices, rapid thermal processing chambers can also contain thermocouples for monitoring the temperature of the wafers. Thermocouples measure the temperature of objects by direct contact.
Many semiconductor heating processes require a wafer to be heated to high temperatures so that various chemical and physical reactions can take place as the wafer is fabricated into a device. During rapid thermal processing, semiconductor wafers are typically heated by arrays of lights to temperatures, for instance, from about 400.degree. C. to about 1,200.degree. C., for times which are typically less than a few minutes and in most cases less than one minute.
Rapid thermal processing chambers typically operate in a non-equilibrium state. Specifically, the walls surrounding the wafer are kept cool, with active cooling, and thus are typically referred to as "cold wall" systems. The non-equilibrium configuration refers to the fact that the filament temperature inside the radiant source is at a much higher temperature than the wafer. The key advantage of using rapid thermal processing systems is the ability to quickly change wafer temperature therefore enabling very short heating cycles which can be between about one second to about two minutes. The drawback of rapid thermal processing systems, however, is the cost per wafer processed, especially for cycles between 60 and 120 seconds long, since the systems are only equipped to heat one wafer at a time.
In view of the above, a need currently exists for a thermal processing chamber that is capable of rapidly heat treating a plurality of wafers at the same time. Specifically, a need exists for a thermal processing chamber capable of accommodating a plurality of wafers as in a batch system, but yet also capable of processing the wafers very rapidly, such as in a rapid thermal processing system.